How did Skulduggery Pleasant come back to life?

When he died he didnt actually move on, although he had died something was holding him in this world, so when he saw that they were losing the war he just came back, like waking up from a vivid dream.

However in mortal coail lord vile is suggesteds to have brought him back

*SPOILER ALERT FOR DEATH BRINGER (BOOK 6)*

In Death Bringer it is revealed by Aoron Tenebrae that Skulduggery is one of the very rare sorcerers who can use more than one disipline of magic as he witnessed saw skulduggery use his necro-imbued dagger without his permission. When he was killed by the poison dagger his own necromancy brought him back but he still had the rage and anger he felt when his family was killed for a short time this fueled skulduggery until one night it completely enveloped him and he went to the necromancers graveyard, walked through the tombstones, knocked on their door and Lord Vile walked into the temple to begin his training.